1. Field of the Invention
The present invention relates to a power-saving low-frequency power amplifier, and more particularly, to a circuit adapted to restrict the power consumption in an amplifier circuit of low frequency power in a system having a waiting condition for an irregular time interval.
2. Description of the Prior Art
Recently, many developments have been made on pocket size receiver, for example, cordless telephones, radio pagers or portable telephones, which can effect wireless communication between a base unit and a handset unit.
The handset unit of the above described type is generally operated by a battery, and usually has a waiting (or stand-by) state during which the circuit power is cut off so as to prevent the power consumption therein, and a operating (or in-use) state during which the circuit is energized to effect the amplification of the received signal. The setting of the stand-by state or operating state is effected by a control signal produced from CPU (central processing unit).
In FIG. 1, a block diagram of a prior art powersaving low-frequency power amplifier is shown, which is employed in a handset unit such as in a handset of cordless telephone. In FIG. 1, the handset unit includes an antenna 1 for receiving electric waves 8 from the base unit (not shown), a processing circuit 2 for processing the received signals, a low frequency power amplifier 3 for amplifying low frequency signal, a speaker 4 connected to low frequency power amplifier 3 through an output coupling capacitor Co, a CPU 5 and a switching circuit 6 coupled with a power source 7 such as a battery. By the control of CPU, the electric power from power source 7 is supplied through switching circuit 6 to processing circuit 2, amplifier 3, CPU 5, and other circuits.
When the handset unit employs an intermittent power saving system, CPU 5 produces the control signal which is a pulse signal having a predetermined frequency. During the pulse period, it is detected whether any data signal is being transmitted to the handset unit, or not, at received signal processing circuit 2. If no data signal is being received, switching circuit 6 continues to provide no electric power to low frequency power amplifier 3. If, on the other hand, data signal is received during the pulse period, CPU 5 controls switching circuit 6 to start providing electric power to low frequency power amplifier 3. Thus, output coupling capacitor Co for DC cutting is charged rapidly. By such a rapid electric charge in the output coupling capacitor Co, speaker 4 produces short but annoying shock noise each time when the low frequency power amplifier 3 is powered.
When the handset unit employs an active/inactive power saving system, CPU 5 produces the control signal which is an ID signal indicating the presence and absence of the data signal from the base unit. When the ID signal from the base unit is not detected, the switching circuit 6 provides no electric power to the low frequency power amplifier 3 to maintain the amplifier 3 in inactive condition. On the other hand, when the ID signal from the base unit is detected, the switching circuit 6 provides electric power to the low frequency power amplifier 3 to switch on the amplifier 3 in active condition. When the low frequency power amplifier 3 is powered on, speaker 4 produces the same annoying shock noise due to the rapid electric charge in the output coupling capacitor Co.
As described above, although the low frequency power amplifier 3 is switched between power on and power off conditions to save the battery power while the handset unit is not in use, undesirable shock noise is produced from the speaker 4 each time the low frequency power amplifier 3 is powered on.